In recent years, various power storage devices such as secondary batteries including lithium-ion secondary batteries and the like, lithium ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electrical devices, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as chargeable energy supply sources for today's information society.
A negative electrode for power storage devices such as lithium-ion secondary batteries and the lithium ion capacitors is a structure body including at least a current collector (hereinafter also referred to as a negative electrode current collector) and an active material layer (hereinafter also referred to as a negative electrode active material layer) provided over a surface of the negative electrode current collector. The negative electrode active material layer contains an active material (hereinafter also referred to as a negative electrode active material) which can occlude and release lithium ions serving as carrier ions and formed of a carbon material, an alloy material, or the like.
At present, a negative electrode containing a graphite-based carbon material is commonly used as a negative electrode for a lithium-ion secondary battery and is formed, for example, in the following manner: graphite as a negative electrode active material, acetylene black (AB) as a conductive additive, and PVDF which is a resin as a binder are mixed to form slurry, the slurry is applied over a current collector, and the slurry is dried.
Such a negative electrode for a lithium-ion secondary battery or a lithium-ion capacitor has an extremely low electrode potential and a high reducing ability. For this reason, an electrolyte solution using an organic solvent is reductively decomposed. The range of potentials in which the electrolysis of an electrolyte solution does not occur is referred to as a potential window. The negative electrode essentially needs to have an electrode potential within the potential window of the electrolyte solution. However, the negative electrode potentials of a lithium-ion secondary battery or a lithium-ion capacitor are out of the potential windows of almost all electrolyte solutions. Actually, a decomposition product of the electrolysis forms a surface film on the surface of the negative electrode, and the surface film inhibits further reductive decomposition. Consequently, lithium ions can be inserted into the negative electrode with the use of a low electrode potential below the potential window of the electrolyte solution (e.g., Non-Patent Document 1).
However, since such a surface film formed of the decomposition product kinetically inhibits the decomposition of the electrolyte solution, deterioration gradually occurs. Therefore, it cannot be said that such a surface film is sufficiently stable. The decomposition reaction speeds up particularly at high temperature; thus, the decomposition reaction hinders operation of a battery in high temperature environments. In addition, the formation of the surface film causes irreversible capacity, resulting in a partial loss of charge and discharge capacity. For these reasons, there is demand for an artificial coating film which is different from the surface film, that is, an artificial coating film on the surface of the negative electrode which is more stable and can be formed without losing capacity.
Further, such a surface film has extremely small electric conductivity, which lowers the electric conductivity of an electrode while a battery is charged and discharged. For this reason, electrode potential distribution is inhomogeneous. Consequently, the charge and discharge capacity of the battery is low, and the cycle life of the battery is short due to local charge and discharge.
On the other hand, at present, a lithium-containing complex phosphate or the like is used as an active material in a positive electrode for a lithium-ion secondary battery. The decomposition reaction between such a material and an electrolyte solution occurs at high temperature and high voltage; accordingly, a surface film is formed due to the decomposition product. Therefore, as in the negative electrode, irreversible capacity is caused in the positive electrode, resulting in a decrease in charge and discharge capacity.
Here, Patent Document 1 discloses that, to prevent deterioration of charge and discharge cycle characteristics and life properties caused by an active material dropping off from a current collector, perhydropolysilazane is used in combination with a binder which is a fluorine macromolecule, and an electrode mix using the perhydropolysilazane, the binder, and a positive electrode material is applied to a current collector and then heated to form an electrode coated with a complex film of the perhydropolysilazane and the binder.
In addition, Patent Document 2 discloses that a carbon particle coated with a thin metal film on its surface or inside by a metal alkoxide treatment method, a sol-gel method, or the like is used to form a negative electrode for a lithium secondary battery with improved cycle characteristics or the like.